1. Field of the Invention
This invention relates generally to adjustments to the stiffness of a golf club shaft. More specifically, the invention relates to a method of adjusting the stiffness, and therefore the natural frequency, of a golf club shaft, and a golf club shaft having a natural frequency that is easily adjusted over a wide range of frequencies.
2. Description of the Related Art
It has recently been recognized that it is advantageous to match a golfer with his or her clubs. As a golfer swings a golf club, he or she applies forces to the handgrip which cause the club shaft to bend. The rate and degree to which each golfer applies forces to the club is unique, and therefore in order to optimize the power and accuracy of the golfer's swing it is desirable to give to that golfer a club having physical properties which are carefully matched to his or her unique swing characteristics.
Some characteristics of a golfer's swing can be quantified, as shown in U.S. Pat. No. 5,351,952, which is incorporated by reference. After quantifying these characteristics, the next step is to obtain a matching club, i.e., a club having physical properties matched to that golfer's unique swing characteristics. The shaft of the golf club is the part that has the most significant effect on a golf club's properties.
Golf club shafts are conventionally made of metal or a composite material, such as graphite fibers in an epoxy matrix. Composite shafts have longitudinal fibers to control longitudinal flexure or stiffness, and helical fibers for controlling torsional stiffness. The shape of most composite shafts gradually tapers along the length from handgrip to club head. The shape of most steel shafts either gradually tapers or tapers in steps, the latter of which is called a "stepped" shaft. A stepped shaft has multiple, distinct sections of different diameter, creating an overall appearance of tapering from a larger to a smaller diameter, but in small, abrupt steps. Stepped composite shafts have also been developed.
The number of potential combinations of golf swing characteristics is very high, resulting in an enormous number of different possible golf clubs. If it is desired that an inventory of finished golf clubs be kept on hand to suit any possible swing characteristics, the inventory would be unfeasibly large and costly. If, on the other hand, each golf club is custom manufactured from golf club parts which are taken from an inventory and can be modified within a spectrum of properties, the inventory necessary to be able to construct each club or set of clubs would still need to be very large. The characteristics of conventional golf club shafts can only be adjusted by a small amount, for example over a spectrum of 30 cycles per minute.
There is therefore a significant need for a method of tailoring the physical characteristics of a golf club to the swing of a golfer from a small inventory of golf clubs. There is also a need for a golf club which can be customized to a golfer's swing quickly and accurately. The natural frequency of such a golf club should be able to be varied over a large spectrum in order to keep the inventory at a minimum.